Survey Design

The baseline survey was conducted in 1998, with follow-up surveys with replacements for deceased elders were conducted in 2000, 2002, and 2005 in a randomly selected half of the total number of counties and cities in the 22 provinces. The survey areas covered 1.1 billion people, 85 percent of the total population in China. An enumerator and a nurse or a medical school student conducted the interview and performed a basic health examination at each interviewee's home.

The CLHLS attempted to interview all centenarians who voluntarily agreed to participate in the study in the randomly selected half of the counties and cities of the 22 provinces. For each centenarian, one near-by octogenarian (aged 80-89) and one near-by nonagenarian (aged 90-99) of pre-designated age and sex was interviewed. “Near-by” is loosely defined – it could be in the same village or street if available, or in the same town or in the same sampled county or city. The predefined age and sex, used to identify the approximately equal numbers of male and female nonagenarians and octogenarians, are randomly determined, based on the code numbers of the centenarians. The goal was to have comparable numbers of male and female octogenarians and nonagenarians at each age from 80 to 99.

We interviewed 9,093 oldest-old persons aged 80+ (including 2,418 centenarians) in our 1998 base line survey, and 11,098 oldest old aged 80+ (including 2,431 centenarians) in our 2000 follow-up survey. Among 11,098, 4,761 elders were survivors who were interviewed in 1998 and re-interviewed in 2000; and 6,337 elders were newly recruited in 2000. In addition, 3,338 valid questionnaire interviews of those who died after the 1998 survey were conducted from a close family member of those deceased elders.

With additional support from UNFPA, the China National Social Science Foundation, and other Chinese resources, the 2002 wave of our survey expanded from covering ages 80+ to ages 65+, with a total sample size of 16,057 elders (including 3,321 centenarians, 3,632 nonagenarians, 4,210 octogenarians, and 4,894 younger elders aged 65-79) plus interviews from the next-of kin interviews for 3,348 respondents who were interviewed in the 2000 wave but died before the 2002 wave.

With supports from the Taiwan Academia Sinica and the China National Foundation for Social Sciences, we have added a sub-sample of 4,478 elderly interviewees’ adult children aged 35-65 in eight provinces in eastern coast areas among the 22 provinces of our healthy longevity survey. The main goal is to comparatively study intergenerational relationships in the context of rapid aging and healthy longevity between China Mainland and Taiwan. The total sample size of our 2002 survey is therefore nearly 21,000 persons aged 35-110.

The 2005 wave interviewed 4,955 young elderly aged 65-79 and 10,658 oldest-old aged 80+ (including 2,797 centenarians, 3,952 nonagenarians and 3,909  octogenarians), with a total sample size of 15,613 (another 25 elders are younger than age 65). Information on before dying was collected from the next-of-kin for nearly 5,900 deceased respondents who were interviewed in 2002 but died before the 2005 wave.

Among the total number of 59,271 interviews with the elderly in the four waves (1998, 2000, 2002 and 2005), 10,750 interviews were for centenarians, 14,038 for nonagenarians, 16,523 for octogenarians, and 10,442 for younger elderly aged 65-79. We also collected data on date/cause of death and disability and suffering before dying through interviews with a close family member for more than 12,500 deceased  elderly. These data combined provide the largest study of the oldest-old ever conducted in the world.